If you use "than" as a preposition, then "her" is the object of that preposition and correctly placed in the objective case. If you use "than" as a conjunction to introduce the subordinate clause "she is [pretty]," then "she" is the subject of the clause and is correctly place in the nominative case. If you elide the "is," this second case will be understood.
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deadratQuoteIf you use "than" as a preposition, then "her" is the object of that preposition and correctly placed in the objective case.If you use "than" as a conjunction to introduce the subordinate clause "she is [pretty]," then "she" is the subject of the clause and is correctly place in the nominative case. If you elide the "is," this second case will be understood.